Social networks are too inclusive and do not permit users to set up multiple accounts under one user login. Users are expected to have a single account and to share posted information with all users, regardless of that user's affiliation to other members. This causes both privacy and propriety concerns. What you would share with a friend is not necessarily what you would share with an elder family member.
Many social networks also delete or lock user accounts when the user passes on or when the user fails some login or use requirement. Users cannot leave a legacy of information for others.
Many sites also do not discriminate security or privacy settings, or permit sharing data selectively. User information is easily hacked, or easily viewed by persons that the user would prefer not to share information with.